Speed vs Rpm question
Burning Brakes
Joined: Feb 2017
Posts: 1,219
Likes: 138
From: Golden, CO
I would like to ask the opposite question.
What are the MINIMUM RPMs that the 991 should be run in each gear. My 74 914, 87 911, 89 911 and 95 993 manuals all clearly articulated minimum RPMs in each gear before the driver is DRAGGING in each gear. My 2013 C2 991 had PDK but my 2015 991 GTS has 7 MT and the manual does not refer to the minima.
Can anyone kindly supply this information that I missed for the MT 991.
Thank you so much for providing the information kindly.
What are the MINIMUM RPMs that the 991 should be run in each gear. My 74 914, 87 911, 89 911 and 95 993 manuals all clearly articulated minimum RPMs in each gear before the driver is DRAGGING in each gear. My 2013 C2 991 had PDK but my 2015 991 GTS has 7 MT and the manual does not refer to the minima.
Can anyone kindly supply this information that I missed for the MT 991.
Thank you so much for providing the information kindly.
So it's just a question of whether you're in a hurry or in traffic conditions where turbo lag is not an issue.
Rennlist Member
Joined: Jan 2016
Posts: 6,207
Likes: 3,533
From: New Orleans, LA (NOLA)
Keep in mind that drag coefficient is only one component of aerodynamic drag. The other is the cross sectional area of the car, known as frontal area. Drag coefficient indicates how smooth a hole you are punching in the air. Frontal area indicates how large that hole is. Total aerodynamic drag is called drag area, and it is Cd x FA.
So the GTS is aerodynamically worse than a base car or an S in two ways. It has both a higher Cd and it also has a larger frontal area, due to the wider rear fenders.
Also, earlier someone had stated that required horsepower goes up with the square of speed. It actually goes up with cube of speed. That's because drag increases with the square of speed, but power is drag times speed, hence the third power.
So the GTS is aerodynamically worse than a base car or an S in two ways. It has both a higher Cd and it also has a larger frontal area, due to the wider rear fenders.
Also, earlier someone had stated that required horsepower goes up with the square of speed. It actually goes up with cube of speed. That's because drag increases with the square of speed, but power is drag times speed, hence the third power.
On the second point, I believe you are are getting laminar and turbulent flow mixed up to come up with the cube. Drag forces due to laminar flow (directly proportional to speed) and turbulent flow (directly proportional to the square of the speed) are not multiplied to obtain a cube of the speed. Also, drag is not the same as the drag coefficient which may be confusing the discussion a bit. Unless I made a mistake in interpreting your post.....entirely possible since I am two drinks in tonight and apparently really really bored. lol.
Back to my Malbec.
Rennlist Member
Joined: Jan 2016
Posts: 6,207
Likes: 3,533
From: New Orleans, LA (NOLA)
I would like to ask the opposite question.
What are the MINIMUM RPMs that the 991 should be run in each gear. My 74 914, 87 911, 89 911 and 95 993 manuals all clearly articulated minimum RPMs in each gear before the driver is DRAGGING in each gear. My 2013 C2 991 had PDK but my 2015 991 GTS has 7 MT and the manual does not refer to the minima.
Can anyone kindly supply this information that I missed for the MT 991.
Thank you so much for providing the information kindly.
What are the MINIMUM RPMs that the 991 should be run in each gear. My 74 914, 87 911, 89 911 and 95 993 manuals all clearly articulated minimum RPMs in each gear before the driver is DRAGGING in each gear. My 2013 C2 991 had PDK but my 2015 991 GTS has 7 MT and the manual does not refer to the minima.
Can anyone kindly supply this information that I missed for the MT 991.
Thank you so much for providing the information kindly.
Burning Brakes
Joined: Feb 2017
Posts: 1,219
Likes: 138
From: Golden, CO
Sorry Valve.....but you are mistaken on both points. The drag coefficient on any object includes both "skin drag" as you have suggested as well as "form drag" which includes the shape of the object. The only question is if the drag coefficient reported by Porsche is with the spoiler up or down. The drag coefficient is ALWAYS associated with the shape of the object or frontal area.
On the second point, I believe you are are getting laminar and turbulent flow mixed up to come up with the cube. Drag forces due to laminar flow (directly proportional to speed) and turbulent flow (directly proportional to the square of the speed) are not multiplied to obtain a cube of the speed. Also, drag is not the same as the drag coefficient which may be confusing the discussion a bit. Unless I made a mistake in interpreting your post.....entirely possible since I am two drinks in tonight and apparently really really bored. lol. Back to my Malbec.
On the second point, I believe you are are getting laminar and turbulent flow mixed up to come up with the cube. Drag forces due to laminar flow (directly proportional to speed) and turbulent flow (directly proportional to the square of the speed) are not multiplied to obtain a cube of the speed. Also, drag is not the same as the drag coefficient which may be confusing the discussion a bit. Unless I made a mistake in interpreting your post.....entirely possible since I am two drinks in tonight and apparently really really bored. lol.
Back to my Malbec.As to the power, the cube function has nothing to do with laminar or turbulent flow. Neglecting Reynolds number effects, which are irrelevant at the automotive speeds we are discussing, the drag of a car = Cd x air density/2 x velocity^2 x Frontal Area. That's where your square term comes from. However, this is a drag FORCE.
POWER = FORCE x VELOCITY. Therefore, there are three velocity terms, two in the drag calculation and one in the power calculation, that determine the power required to achieve any particular speed. Hence the cubed term.
